John the savage says to mustapha mond, "you paid a fairly high price for your happiness." what does he mean by this? what are the things that mond had to sacrifice in order to for society to be "happy?"

The answer is:

In Aldous Huxley's Brave New World when John the savage says to Mustapha Mond, "you paid a fairly high price for your happiness." what he means by this is best summarized in the following expression by Goethe, "None are more hopelessly ensalved than those who falsely believe they are free."

Mustapha Mond had to sacrifice religion, happiness and art along with many other things in order for society to be "happy".

Explanation from Chapter 17

Religion is the very last sacrifice made by the world of old to ensure happiness. Mond explains that since society has eradicated the fear of death and in this Brave New World science keeps everyone youthful until death, religion is unnecessary. In turn, John The Savage complains that this new society is preventing people from discovering truth for themselves. John Savage argues that religion comes naturally to man and will never entirely disappear. The religion of the Indian Tribes gives great meaning to their lives and provides the ability to endure turmoil and even be able to experience unhappiness.

Mustapha and John The Savage then argue over the meaning of life and happiness. Mond tells John Savage, "In fact, you're claiming the right to be unhappy." In the extremism of the Utopian New World, the right to be unhappy no longer exists! John The Savage argues that being unhappy is a natural right that every man should be able to experience. Mustapha disagrees with him.